The present invention relates to isolator diaphragms made of materials which must be anchored to an isolator housing formed of other materials which are incompatible with direct welding to the isolator diaphragm. A tantalum diaphragm, for example, has a very high melting point while a stainless steel isolator housing has a lower melting point and it is not practical to weld the two parts directly to one another.
Isolating diaphragms are used in pressure transmitters to couple pressure from process fluids outside the transmitter to an isolator fluid sealed inside the transmitter. The isolator fluid, in turn, couples the pressure to a pressure sensor. Process fluids, which can be highly corrosive, are thus kept isolated from the pressure sensor to avoid corrosion or damage to the pressure sensor.
Various corrosion resistant, high cost metals are used for the diaphragms. Tantalum is a material that is very resistant to corrosion, but it has a considerably higher melting point than other materials that are conventionally used for isolator diaphragms, such as 316 L stainless steel, Hastalloy C and Monel. The isolator housing mounting the isolating diaphragm may be made of various stainless steel alloys which have a much lower melting point than tantalum.
A welded joint around the rim of the isolator diaphragm is preferred, however, welding of a tantalum isolator diaphragm directly to the isolator housing isn't practical because the isolator housing melts at a much lower temperature than the tantalum isolator. The term "welding" means joining two metals parts together by applying heat to melt both metal parts near mating surfaces. Welding is thus distinguished from brazing in which at least one of the parts being joined does not melt significantly and there is a filler metal with a lower melting temperature introduced in the braze joint.
An isolator diaphragm can also be incompatible with direct welding to an isolator housing of other material because the weld contaminates the isolator diaphragm with the other metal from the housing. Dissimilar metals in the isolator diaphragm then contact the fluid and the corrosion resistance of the isolator diaphragm is reduced.
Prior systems are known for brazing support rings to tantalum isolators. But it is difficult to completely seal the braze joints from contact with the process fluid or its vapors. The braze joint is typically more prone to corrosion than a welded joint would be because of the presence of dissimilar metals in the braze joint. Even small amounts of corrosion can result in a leak because the isolator diaphragm must be kept relatively thin to reduce pressure drop when it is deflected.
There is thus a need for an arrangement which provides a weld to join an isolator diaphragm formed from a corrosion resistant metal such as tantalum to an isolator housing which is formed of other metal not compatible with welding to the isolator diaphragm metal.